Design and Fabrication of Optical Metamaterials and Their Nano Manufacturing Methods

by

Yong-Shik Park

A dissertation submitted in partial satisfaction of the

requirements for the degree of

Doctor of Philosophy

in

Engineering-Mechanical Engineering

in the

Graduate Division

of the

University of California, Berkeley

Committee in charge:

Professor Xiang Zhang, Chair

Professor Costas Grigoropoulos

Professor Ming Wu

Fall 2011

Design and Fabrication of Optical Metamaterials and Their Nano Manufacturing Methods

© 2011

by

Yong-Shik Park

1

Abstract

Design and Fabrication of Optical Metamaterials and Their Nano Manufacturing Methods

by

Yong-Shik Park

Doctor of Philosophy in Engineering - Mechanical Engineering

University of California, Berkeley

Professor Xiang Zhang, Chair

Metamaterials have opened a new field of optics in science and engineering. Negative

refraction has been achieved by manipulating permittivity and permeability to be negative using

the combination of SRRs (split ring resonators) and metal wires and the fishnet structure.

However, the fishnet design suffers two significant disadvantages: anisotropy and scalability.

The fishnet metamaterial needs a specific polarization direction because of the limitations of

anisotropic design. In addition, the conventional projection photolithography approach to

nanoscale manufacturing is facing possibly insurmountable challenges in economically

producing the next generation of semi-conductor integrated circuits. Maskless nanolithography is

a potentially agile and cost effective approach, but most of the current solutions have

throughputs that are too low for viable manufacturing purposes. This dissertation presents novel

isotropic and chiral metamaterials as well as a plasmonic lithography approach to mass

metamaterial manufacturing.

This dissertation reports two new designs of metamaterial which have a negative index of

refraction and overcome the disadvantage of conventional metamaterials. The design of a chiral

metamaterial is proposed, fabricated, and measured to prove negative index of refraction.

Furthermore, a tunable chiral metamaterial is proposed, which is able to switch left and right

circular polarized light dynamically by an external control beam. Also shifted bars and rings

create a negative refractive metamaterial which is isotropic and randomly distributed. In this

thesis, two shifted rings structure are proved to have a negative index of refraction independent

of the direction of polarized light or without polarization. Beside applications of negative

refraction, other applications are studied. The split ring resonator is one of the basic structures of

metamaterial and this structure can be used to measure mode volume and single molecule

detection by far field method.

For the nano manufacturing method, using the plasmonic nanolithography (PNL) approach,

22-nm half-pitch direct pattern writing was successfully demonstrated using ultra-fast laser

assisted nanoscale heat management and advanced plasmonic airbearing flying head designs. In

addition, the details of micro and nano fabrication methods are introduced and they cover all of

the structures proposed in this thesis.

2

To my family, Chanyoung, Chanu, and Jaewon,

And my parents

i

Contents

List of Figures ....................................................................................................................................... iv

List of Tables ........................................................................................................................................ xi

Chapter 1 Introduction ......................................................................................................................... 1

1.1 Introduction to metamaterial ..................................................................................................... 1

1.1.1 Fundamentals of metamaterial .......................................................................................... 1

1.1.2 Negative refraction ........................................................................................................... 2

1.1.3 Negative permittivity and permeability ............................................................................. 3

1.1.4 Chiral metamaterial .......................................................................................................... 5

1.1.5 Parameter retrieval of chiral media ................................................................................... 6

1.2 Introduction to Surface Plasmon Lithography ........................................................................... 9

1.2.1 Fundamental Physics of Surface Plasmon ......................................................................... 9

1.2.2 Plasmonic Nearfield Lithography ................................................................................... 12

1.3 Dissertation Organization ....................................................................................................... 14

Chapter 2 Chiral Metamaterial .......................................................................................................... 15

2.1 Terahertz Chiral Negative-Index-Metamaterials ..................................................................... 15

2.1.1 Background .................................................................................................................... 15

2.1.2 Design of THz Chiral Metamaterial ................................................................................ 15

2.1.3 Fabrication ..................................................................................................................... 16

2.1.4 Experiment ..................................................................................................................... 18

2.1.5 Results and discussion .................................................................................................... 19

2.2 Photo-Induced Chirality Switching in Metamolecules ............................................................. 22

2.2.1 Background .................................................................................................................... 22

2.2.2 Design of THz switchable chiral metamaterial ................................................................ 23

2.2.3 Fabrication ..................................................................................................................... 25

2.2.4 Simulation and Experiment Results ................................................................................ 27

2.2.5 Summary and discussion ................................................................................................ 31

Chapter 3 Asymmetric Isotropic Metamaterial ................................................................................ 32

3.1 Symmetry Breaking in 3D and Optical Negative Index with Closed Nanorings ....................... 32

3.1.1 Background .................................................................................................................... 32

3.1.2 Theory and Design ......................................................................................................... 33

ii

3.1.3 Fabrication and Experimental results .............................................................................. 34

3.1.4 Conclusion ..................................................................................................................... 41

3.2 Negative Index in Random Optical Media .............................................................................. 42

3.2.1 Introduction .................................................................................................................... 42

3.2.2 The system and theory .................................................................................................... 43

3.2.3 Fabrication and Optical Experiment ................................................................................ 45

3.2.4 Conclusion ..................................................................................................................... 48

Chapter 4 Metamaterial Applications ................................................................................................ 49

4.1 Far-field extraction of Deep Sub-wavelength Plasmonic Mode Volume .................................. 49

4.2 Split Ring Resonator Sensors for Infrared Detection of Single Molecular Monolayers ............ 55

Chapter 5 Surface Plasmon Lithography ........................................................................................... 61

5.1 Plasmonic Nano Lithography System ..................................................................................... 61

5.2 Design and fabrication of plasmonic lens................................................................................ 63

5.2.1 Design of H-shaped PLs ................................................................................................. 63

5.2.2 Fabrication of H-shaped PLs ........................................................................................... 64

5.2.3 H-shaped PL with a Ring Reflector ................................................................................. 70

5.2.4 Recessed H-shaped PL for Direct Line Patterning ........................................................... 72

5.2.5 Push-Pin PL ................................................................................................................... 76

5.3 Design and fabrication of flying head ..................................................................................... 79

5.4 Fabrication of media disks ...................................................................................................... 84

5.5 Results and discussion ............................................................................................................ 85

5.5.1 Continuous Wave (CW) Laser Based Tests ..................................................................... 85

5.5.2 Ultrafast Laser Based Tests ............................................................................................ 88

Chapter 6 Fabrication Techniques .................................................................................................... 90

6.1 Terahertz Chiral Metamaterial ................................................................................................ 90

6.1.1 Substrate preparation ...................................................................................................... 90

6.1.2 Deposition of electrode ................................................................................................... 90

6.1.3 Definition of Au pads ..................................................................................................... 91

6.1.4 Definition of contacts ..................................................................................................... 92

6.1.5 Definition of SU-8 mold ................................................................................................. 92

6.1.6 Electroplating ................................................................................................................. 93

6.1.7 Definition of Au bridge .................................................................................................. 94

6.1.8 Post electroplating and dicing ......................................................................................... 94

iii

6.1.9 Release of structures ....................................................................................................... 95

6.2 Terahertz Switchable Chiral Metamaterial .............................................................................. 96

6.2.1 Substrate preparation ...................................................................................................... 96

6.2.2 Definition of Si pads ....................................................................................................... 96

6.2.3 Deposition of electrode ................................................................................................... 97

6.2.4 Definition of Au pads ..................................................................................................... 97

6.2.5 Definition of contacts ..................................................................................................... 98

6.2.6 Definition of SU-8 mold ................................................................................................. 99

6.2.7 Electroplating ................................................................................................................. 99

6.2.8 Definition of Au Bridge ................................................................................................ 100

6.2.9 Post electroplating, release and dicing ........................................................................... 101

6.2.10 Release of structures ..................................................................................................... 102

6.3 High resolution EBL lift-off techniques ................................................................................ 103

6.3.1 Cold development ........................................................................................................ 103

6.3.2 Bi-layer MMA-PMMA resists process .......................................................................... 105

6.3.3 Bi-layer resist process: PMMA-MMA .......................................................................... 107

6.4 High precision EBL alignment ............................................................................................. 109

6.4.1 Location of alignment marks ........................................................................................ 109

6.4.2 Alignment offsets ......................................................................................................... 111

6.5 Multilayer EBL technique .................................................................................................... 113

6.5.1 Interlayer: SU-8 ............................................................................................................ 113

6.5.2 Planarization ................................................................................................................ 114

6.5.3 Process Results ............................................................................................................. 116

6.6 Random Bar Generation ....................................................................................................... 117

Chapter 7 Summary .......................................................................................................................... 127

Bibliography ...................................................................................................................................... 128

iv

List of Figures

Fig. 1. 1: Orientation of field quantities E, H, Poynting vector S, and wavevector k is right-

handed media (RHM) and left-handed media (LHM) .................................................... 1

Fig. 1. 2: Refraction at a two-medium interface as determined by phase matching. The support of

backward waves by an LHM insists on negative refraction (Case 2). ........................... 2

Fig. 1. 3: The transmission and reflection coefficients of a plane wave incident upon a chiral slab

from the left .................................................................................................................... 7

Fig. 1. 4: The charges and the electromagnetic field of SPs propagating on a surface in the x

direction are shown schematically. .............................................................................. 10

Fig. 1. 5: The SP dispersion relation. The dotted line represents the light line where the SP

dispersion curve always lies right of the light line. ...................................................... 10

Fig. 1. 6: Momentum matching through the use of grating coupler ............................................. 11

Fig. 1. 7: Tip side view and cross-sectional view of geometry ((a) and (b)) and simulated

intensity of the nearfield ((c) and (d)) of the plasmonic structures. The scale bars is

500nm. .......................................................................................................................... 12

Fig. 1. 8: The configuration of nearfield lithography and AFM image of the photoresist after

near-field scanning exposure using the plasmonic structures ...................................... 13

Fig. 2. 1: (a) The schematic of the chiral structure, with some of the dimensions indicated in the

figure: L =18m, h=5m, r=1.6m, w=4.4m, g=2.3m. The thicknesses of the

bottom gold strips and the top gold bridge are 0.6 and 0.3 m, respectively. (b) The

top view of the chiral structure, which functions effectively as an electric dipole and a

magnetic dipole forming an angle =29.2o. ................................................................. 16

Fig. 2. 2 (a)-(f): the schematics of the fabricaton procedures for the chiral metamaterials. (g), (h):

the SEM images of the chiral metamaterials at tilted angle. The size of the unit cell is

40 m by 40 m. The scale bars in (g) and (h) represent 100 m and 20 m

respectively. .................................................................................................................. 17

Fig. 2. 3: The time domain terahertz spectroscopy setup to characterize the transmissions of the

chiral metamaterials. The photoconductive switch-based THz-TDS system consists of

four paraboloidal mirrors arranged in an 8-F confocal geometry, enabling excellent

terahertz beam coupling between the transmitter and receiver and compressing the

beam to a frequency independent beam waist with a diameter of 3.5 mm as well. The

THz-TDS system has a usable bandwidth of 0.1-4.5 THz (3 mm-67m) and a signal to

noise ratio over 4 orders of magnitude. ........................................................................ 18

Fig. 2. 4: The time-domain terahertz measurements of the chiral metamaterials. (a) The

amplitudes of transmission t1 and t2 for the polarizer P2 parallel to P1 (black curve)

and P2 perpendicular to P1 (red curve), respectively. (b) The relative phases of

transmission t1 (black) and t2 (red). (c) The derived transmission amplitudes for left

handed (black) and right handed circularly polarized waves (red). (d) The derived

v

relative transmission phases for left handed (black) and right handed circularly

polarized waves (red). .................................................................................................. 19

Fig. 2. 5: (a)-(d): the FDTD simulation of the transmission amplitudes and relative phases for the

chiral metamaterials, with the same color conventions as in Fig. 2. 4. (e) The real part

of refractive indices for right handed (red) and left handed (blue) circularly polarized

waves. (f) The real part of effective permittivity (blue) and permeability (red). ......... 21

Fig. 2. 6: Schematic of the design of chiral switching metamaterials. (a) an achiral metamolecule

consisting of two meta-atoms of opposite chiralities, with the corresponding

anticipated circular dichroism spectra. The equivalent circuit of the metamolecule is

shown on the right. (b) Silicon pads are incorporated to the metamolecule. The mirror

symmetry is broken, and the metamolecule exhibits chirality at the resonance

frequencies. The silicon pads correspond to the switches in the equivalent circuit. (c)

Under optical illumination, photo-carriers are generated, leading to a switching of

chirality in the form of reversed circular dichroism. In the equivalent circuit, the

switches are closed under the optical illumination. ...................................................... 24

Fig. 2. 7: Fabrication procedures of switchable chiral metamaterial. (a)-(g): the schematics of the

fabricaton procedures for the chiral metamaterials. (h), (i): the SEM images of the

switchable chiral metamaterials at tilted angle. ............................................................ 26

Fig. 2. 8: Numerical study on chiral switching metamaterials. (a) the transmission spectra of left

handed (black) and right handed (red) circular polarizations, without (solid) and with

(dashed) optical irradiation. (b) The ellipticity calculated from the transmission spectra

without (black solid) and with pumping (red solid). The shaded area represents the

frequency range where the circular dichroism is reversed. (c) The optical rotatory

dispersion (ORD) without (black solid) and with pumping (red solid). (d, e, f) The

polarization states of transmission, without (black solid) and with (red solid) optical

irradiation, for a linearly polarized wave passing through the chiral switching

metamaterial at 0.836 THz, 1.011 THz and 1.067 THz. .............................................. 28

Fig. 2. 9: Characterizations of chiral switching metamaterials. (a) the transmission spectra of left

handed (black) and right handed (red) circular polarizations, without (solid) and with

(dashed) optical irradiation. (b) The ellipticity calculated from the transmission spectra

without (black solid) and with pumping (red solid). The shaded area represents the

frequency range where the circular dichroism is reversed. (c) The polarization states of

transmission for at 1.142 THz. (d) The measured optical rotatory dispersion (ORD) (e,

f) The polarization states of transmission at 1.054 THz and 1.215 THz. ..................... 30

Fig. 3. 1: (A) Schematic of the coupled closed nanorings. (B) Hybridization scheme of the

metallic nanorings depending on their alignment. When the rings are aligned

(dx=dy=0), the antisymmetric mode corresponds to the bonding mode and occurs at

lower energy than the symmetric mode, ω- aligned < ω

+aligned. The coupling between the

nanorings is positive. When the nanorings are fully shifted with dx=dy=L, the

antisymmetric mode now corresponds to the antibonding mode and occurs at higher

energy than the symmetric mode, ω-shifted > ω

+ shifted , the hydridization is inverted and

the coupling between the nanorings is negative. The change in the modes ordering is

vi

due to the modification of the near field interaction between the nanorings and we

demonstrate that a three dimensional medium built from negatively coupled nanorings

built a 3D negative index metamaterial. ....................................................................... 34

Fig. 3. 2: Measured transmission spectra and normal view scanning electron microscopy images

(background of the graphs) as a function of symmetry breaking. Square lattices

(period=600nm) of coupled square rings fabricated on glass substrate (nglass=1.5) with

L=300nm, w=60nm, t=30nm. The spacer between the two rings layers, SU8 (nSU8=1.5)

is vertically separating the ring layers. Two interlayer thicknesses of s=150nm (thick)

and s=50nm (thin) are considered. Symmetry breakings with ydyxdxvd

are

investigated so that the metamaterials are polarization independent at normal incidence.

Spacer thickness of 150nm (thick): dx=dy=0nm (A) and dx=dy=300nm (B). The

symmetric and antisymmetric modes are excited in the case of aligned rings while in

the shifted configuration the modes become degenerate. Spacer thickness of 50nm

(thin): dx=dy=0nm (C) and dx=dy=300nm (D). The modes are excited in the aligned

configuration with ω-s=50, shifted < ω

+s=50, shifted while in the shifted configuration ω

-s=50,

shifted > ω+

s=50, shifted , the hybridization is inverted. ........................................................ 35

Fig. 3. 3: Interferometric measurements of the broken symmetry coupled nanorings. (A)

Schematic of the Michelson interferometer. (B) Sketch of a side view of the two layer

sample, showing the thickness of the metamaterial (dmeta=110nm) as well as the

reference (dref=80nm). (C) Interferograms measured on the sample and the reference at

1900nm wavelength, showing phase shift corresponding to a negative index of n=-0.93.

(D) Interferograms measured on the sample and the reference at 2000nm wavelength,

showing a phase shift corresponding to an index of +0.65. ......................................... 37

Fig. 3. 4: (A) Tilted view electron micrograph of the three dimensional structure (ten layers) of

stacked broken symmetry nanorings. The dimensions are those of figure 1C with the

last layer also covered by SU8. (B) Sketch of the corresponding metamaterial. ......... 38

Fig. 3. 5: Transmission spectra and dispersion of multilayers of broken symmetry nanorings. (A)

Simulated transmission spectra for different number of rings layer from two to five. (B)

Measured transmission spectra for different number of metamaterial layers. Excellent

agreement between experiment and theory is found. The formation of the passband

between 1.3μm and 2.3μm can also be observed. (C) Effective metamaterial

parameters and transmission for 20 layers of rings. The black and red curves (left axis)

are the real and imaginary parts of the effective refractive index. The blue curve (right

axis) is the transmission of the corresponding metamaterial. The index is negative in

the entire “Fano induced” passband. (D) Time evolution (time increases from left to

right and top to bottom) of uniform phase fronts in the bulk metamaterial showing

backward waves at λ=1.6μm. S and k are the poynting vector and wavevector

respectively. .................................................................................................................. 40

Fig. 3. 6 (a) System under investigation consisting of two gold metallic bars shifted in the

direction of their long wavelength electric dipole moment. (b) Resonance splitting

(symmetric mode ω+ and anti-symmetric mode ω-) from simulations of the coupled

bars as a function of the shift between the strips for a vertical distance between the

bars of S=50nm with particles in the configuration of our experiment. The dimensions

of the bars are L=290nm; w=60nm; t=30nm. The insets indicate the typical mode

vii

pattern (XZ cross section) or charges distribution for the zero and fully shifted bars at

their symmetric and anti-symmetric resonances. ......................................................... 44

Fig. 3. 7: Scanning electron microscopy images of the meta-isotopes 1 to 4 (for density D3) with

different shifts between the bars and experimental measurements. Normal views for

0nm shift (a), 145nm shift (b), 203nm shift (c) and 290nm shift (d). Experimental

transmissions spectra with unpolarized light at normal incidence for the meta-isotope 1

(e), meta-isotope 2 (f), meta-isotope 3 (g) and meta-isotope 4 (h) and for different

filling fractions of the meta-isotopes. ........................................................................... 46

Fig. 3. 8: Experimental transmission at normal incidence for two arbitrary orthogonal

polarizations (0deg and 90 deg) and an intermediate one (45deg) of meta-isotope 1 (a)

and meta-isotope 3 (b) and a density of D3=5.73.108cm

-2. Insets: Magnified views of

the random samples and arbitrary linear polarizations of light. ................................... 47

Fig. 4. 1: Far field measurement of the spectra of loop antenna arrays. a. The schematic of a loop

optical antenna. The dimensions are indicated in the figure: The width of the ring W

=68 nm, height of ring h=40 nm, outer radius r=155 nm, and the average gap g ranges

from 24.1 nm to 54.8 nm for different samples. b. the SEM image of an loop antenna

array with a period of 600 nm. c. the SEM images of four different samples with

average gap widths of 24.1 nm, 32.1 nm, 45 nm and 54.8 nm, respectively. d. The

measured transmission spectra using Fourier transform infrared spectroscopy. Black,

red, cyan and blue curves correspond to samples with increasing gap sizes. .............. 51

Fig. 4. 2: Simulated spectral response of the loop antenna arrays with different gap widths. The

color convention is the same as in Fig. 4.1(d). The inset shows the electric field

distribution at the resonance frequency (f = 120 THz) for the loop antenna with gap

width of 24.1 nm. The electric field is strongly confined inside the gap of the antenna,

while the magnetic field is enhanced inside the circular loop due to the oscillating

current in the antenna. .................................................................................................. 52

Fig. 4. 3: Measured and simulated mode volumes of loop antennas. The black diamond and blue

square represent the mode volume extracted from resonance frequencies obtained from

measurement and Microwave Studio simulation, respectively. The red solid line

corresponds to the mode volume calculated rigorously from the near field electric and

magnetic field distribution of the eigen mode. For comparison, the geometrical gap

volume, calculated as gwhVgap , is shown as the gray line. ...................................... 53

Fig. 4. 4 (a) Scanning electron micrograph (SEM) of a typical split ring resonator (SRR) array.

Scale bar is 1 μm. Inset shows a schematic of a self-assembled monolayer of ODT

molecules in the gap of a single SRR for absorption spectroscopy. (b) Simulated near-

field amplitude distribution around a SRR on resonance for polarization along the gap.

Near-field is confined mostly in the gap. ..................................................................... 56

Fig. 4. 5: (a) Calculated near-field spectrum in the SRR gap with a uniform 2.4 nm layer of a

Lorentzian absorber on SRRs to emulate the experiments. (b) Calculated far-field

transmission spectrum for the same configuration. Inset is a closer view of the

molecular absorption peak superimposed on the SRR spectrum. ................................ 57

viii

Fig. 4. 6: Measured transmittance for five SRR arrays with different diameters for polarization

along the gap. Vertical lines represent the position of ODT absorption peaks relative

to different SRR spectra. .............................................................................................. 58

Fig. 4. 7: (a) Measured transmittance spectra for two different SRR arrays with radii 170 nm

(blue), 180 nm (red), and 210 nm (green) with the SAM. Two peaks corresponding to

ODT absorption are clearly visible. (b) Close-up view of the resonance region in (a).

...................................................................................................................................... 59

Fig. 5. 1: A schematic of the PNL experimental setup. The PL focuses ultraviolet laser pulses

onto the rotating substrate by concentrating surface plasmons (SPPs) into nanoscale

spots. An advanced airbearing surface (ABS) technology is used to maintain the gap

between the lens and the substrate at 10 nm. A pattern generator is used to pick the

laser pulses for exposure through an optical modulator according to the angular

position of the substrate from the spindle encoder and the radial position of the flying

head from a nanostage. ................................................................................................. 62

Fig. 5. 2: Snap-shot of PNL base system ...................................................................................... 62

Fig. 5. 3: Typical H-shaped PLs. (a) The structure of one PL: H-aperture surrounded by two

rings. The inset shows the parameters of the H-aperture. (b) E field intensity

distribution at the plane 25 nm away from the PL. ...................................................... 64

Fig. 5. 4: Effect of writing beam currents in FIB; (a) 4pA, (b) 70pA........................................... 65

Fig. 5. 5: Serial and parallel writing in FIB .................................................................................. 66

Fig. 5. 6: Effect of magnification in FIB ...................................................................................... 67

Fig. 5. 7: Dimensional differences between input and pattern width ........................................... 68

Fig. 5. 8: Parameter matrix with width and gap of antenna .......................................................... 69

Fig. 5. 9: The optimized input parameter of FIB and the final image of H-shaped aperture ........ 70

Fig. 5. 10: An example of H-shaped PL. (a) The structure of an improved PL design: H-aperture

surrounded by two rings and a reflector ring. (b) E field intensity distribution at the

plane 25 nm away from the PL. ................................................................................... 71

Fig. 5. 11: Poynting vector fields at the plane 25 nm away from the PLs. (a) The PL with two

rings only (b) The PL with two rings and a reflector. The insets are the enlarged views

of the pointing vector fields at the regions immediately outside the last ring of the two

PLs. ............................................................................................................................... 71

Fig. 5. 12: SEM pictures of H-shaped PL with a ring reflector. ................................................... 72

Fig. 5. 13: Recessed H-shaped PL design and performance. (a) Lens dimensions. (b) Light

intensity profile at 15 nm distance from the PL. (c) and (d) Cross-section profiles. ... 73

Fig. 5. 14: Test parameters to calibrate the etch depth ................................................................. 74

Fig. 5. 15 AFM image of the recessed H-shaped PL .................................................................... 75

Fig. 5. 16: SEM picture of recessed H-shaped PL. ....................................................................... 75

ix

Fig. 5. 17: An example of push-pin design using gold for the wavelength of 633 nm. (a) The

dimensions of the PL design. (b) The base surface of the PL is placed 30 nm away

from the other gold surface which leaves a clearance of 10 nm at the position of the

pin. (c) An enhancement of 2000 times is achieved at the distance of 5 nm away from

the pin. (d) At a distance of 10 nm where the second gold surface is, the enhancement

factor is about 800 times. ............................................................................................. 77

Fig. 5. 18: Fabricated gold push-pin PL structure fabricated and its nearfield aperture-less NSOM

studied. (a) SEM image. (b) AFM measured lens profile. (c) and (d) Superposed AFM

measurement with aperture-less NSOM measurement. ............................................... 78

Fig. 5. 19: The ABS design (a) Oblique view. The topography is scaled up by 200 times for

better illustration. (b) Normal air pressure and mass flow lines under ABS. The

pressure is normalized to ambient air pressure. ........................................................... 80

Fig. 5. 20: FH measurement of fabricated plasmonic flying head. (a) Measured and calculated

FH shows the slider maintains the FH at 20 nm, with scanning speeds between 4 and

12 m/s. (b) Measured and simulated pitch and roll angles at scanning speeds between 4

and 12 m/s. Agreement between experiment and simulation demonstrates that the

parallelism achieved is within the gap tolerance of 30 nm over the whole area of the

PL array and the substrate. ........................................................................................... 81

Fig. 5. 21: ABS fabrication procedures ........................................................................................ 83

Fig. 5. 22: Maskless Lithography by flying PLs at near field. (a) AFM image of pattern with 80

nm line width on the TeOx based thermal photoresist. (b) AFM image of arbitrary

writing of “SINAM” with 145 nm line width. (c) Optical micrograph of patterning of

the large arrays of “SINAM”. ....................................................................................... 86

Fig. 5. 23: A dual-spot PL for sub-30 nm lines writing. Top-left figure shows the lens geometry.

Under plane wave illumination, this design produces two hot spots (top-middle) and

with off-center illuminations (bottom-left) it produces one very narrow single elliptical

hot spot (bottom-middle). The right figure shows the AFM image of the lithography

result of semi-dashed lines modulated between 20 nm and 30 nm in width and 1 μm in

period. ........................................................................................................................... 87

Fig. 5. 24: AFM images of (left) a group of 50 nm wide lines with 20 nm depth ........................ 88

Fig. 5. 25: PNL results. (a) The AFM image of closely packed dots with a 22 nm half pitch. (b)

The AFM image of closely packed dots with 27 nm half pitch size. (c) The AFM

image of arbitrary pattern writing at 30 nm linewidth. ................................................ 89

Fig. 6. 1:Silicon oxide on Si substrate .......................................................................................... 90

Fig. 6. 2: Deposition of electrode.................................................................................................. 91

Fig. 6. 3: Definition of Au pads .................................................................................................... 91

Fig. 6. 4: Definition of contacts .................................................................................................... 92

Fig. 6. 5: Definition of SU-8 mold ................................................................................................ 93

Fig. 6. 6: Electroplating of pillars ................................................................................................. 93

x

Fig. 6. 7: Definition of Au bridges ................................................................................................ 94

Fig. 6. 8: Release of the structure ................................................................................................. 95

Fig. 6. 9 Silicon on sapphire substrate .......................................................................................... 96

Fig. 6. 10: Definition of Si pads: (a) After Si pads, (b) Optical Microscope image .................... 97

Fig. 6. 11: Deposition of electrode ................................................................................................ 97

Fig. 6. 12: Definition of Au pads .................................................................................................. 98

Fig. 6. 13: Definition of contacts .................................................................................................. 99

Fig. 6. 14: Definition of SU-8 mold .............................................................................................. 99

Fig. 6. 15: Electroplating of pillars ............................................................................................. 100

Fig. 6. 16: Definition of Au bridges ............................................................................................ 100

Fig. 6. 17: Release of the structure ............................................................................................. 102

Fig. 6. 18: The effect of the temperature of a developer148; (a) Room temperature, (b) Low

temperature ................................................................................................................. 103

Fig. 6. 19: The comparison of the temperature of developer ...................................................... 104

Fig. 6. 20: Examples of the structure made by cold development process ................................. 104

Fig. 6. 21: The process flow of bi-layer process: MMA-PMMA ............................................... 105

Fig. 6. 22: Example of bi-layer MMA-PMMA process ............................................................. 106

Fig. 6. 23: The process flow of bi-layer process: PMMA-MMA ............................................... 107

Fig. 6. 24: Example of bi-layer PMMA-MMA process ............................................................. 108

Fig. 6. 25: Locations of alignment marks ................................................................................... 109

Fig. 6. 26: Alignment test pattern ............................................................................................... 110

Fig. 6. 27: Mis-alignment offsets for x and y directions ............................................................. 111

Fig. 6. 28: Mis-alignment with the optimized conditions ........................................................... 112

Fig. 6. 29: Thickness by spin speed with respect to the volume percent dilution ...................... 113

Fig. 6. 30: Resist thickness changes after reflow ........................................................................ 114

Fig. 6. 31: Surface profile change after planarization ................................................................. 115

Fig. 6. 32: Comparison of the interlayer of SiO2 and the interlayer of SU-8 ............................. 116

Fig. 6. 33: Generation a new bar and transformation of coordination ........................................ 118

Fig. 6. 34: Modes of duplications ............................................................................................... 119

Fig. 6. 35: Criteria of overlaps .................................................................................................... 121

Fig. 6. 36: Definition of a minimum gap .................................................................................... 124

Fig. 6. 37: Comparison of overlaps ............................................................................................. 125

Fig. 6. 38: Comparison of the different filling ratios .................................................................. 126

xi

List of Tables

Table 4. 1: Measured gap widths and resonance frequencies ....................................................... 52

Table 5. 1: Process parameters of FIB .......................................................................................... 64

Table 5. 2: Process parameters and the width and length of one side of antenna ......................... 69

Table 5. 3: Process parameter to calibrate etch depth ................................................................... 74

Table 6. 1: Standard deviations of different locations of alignment marks ................................ 111

Table 6. 2: Mis-alignment with the optimized conditions .......................................................... 112

Table 6. 3: Characteristics of SU-8 2000 .................................................................................... 113

1

Chapter 1 Introduction

1.1 Introduction to metamaterial

1.1.1 Fundamentals of metamaterial

In the 1960‟s, Veselago showed the feasibility of a media characterized by a simultaneously

negative permittivity and permeability[1] and it is allowed by Maxwell‟s equations and that

plane waves propagating inside them could be described by an electric field intensity vector E,

magnetic field vector H, and wavevector for k, forming a left-handed triplet, in seeming

opposition to wave propagation in conventional media, in which these three quantities form a

right-handed triplet, and accordingly labeled theses materials left-handed media (LHM) and

right-handed triplet media (RHM), respectively. The two arrangements are shown in Fig. 1. 1.

Although E, H, and k form a left-handed triplet, E, H, and the Poynting vector S maintain a right-

handed relationship. Therefore, the wavevector k in LHM is the other direction of the Poynting

vector S. It is described as the backward wave by Veselago. For this reason, the term „backward

wave‟ is used to describe left-handed media [2]. What is remarkable in Veselago‟s work is that

two or three dimensional isotropic and homogeneous media supporting backwards waves ought

to be characterized by a negative refractive index, which is defined by taking the negative branch

of the square root in the definition, . Consequently, when such media are interfaced with conventional dielectrics, Snell‟s law is reversed, leading to the negative refraction of an

incident electromagnetic plane wave, Such a material, in realized form, could appropriately be

called a metamaterial, where the prefix meta, Greek for „beyond‟ or „after‟, suggests that is

possesses properties that transcend those available in nature[3].

(a) RHM (b) LHM

Fig. 1. 1: Orientation of field quantities E, H, Poynting vector S, and wavevector k is right-

handed media (RHM) and left-handed media (LHM)

2

1.1.2 Negative refraction

Through the idea of phase matching, negative refraction can be understood. To illustrate,

consider the two-medium interface of Fig. 1. 2, where medium 1 (M1) is an RHM and medium 2

(M2) is unspecified for the moment. A plane wave originating in M1 is incident of the interface

with wavevector k1, and it establishes a refracted wave in M2 with wavevector k2 such that their

tangential components k1t and k2t are equal, according to the conservation of the wave

momentum. Having specified the tangential components, we immediately recognize that there

are two possibilities for the normal component of k2: the first case, in which k2 is directed away

from the interface, and the second case, usually describing reflected waves, in which k2 is

directed towards the interface. There two cases are represented as Case 1 and Case 2 in Fig. 1. 2.

By the conservation of energy, the normal components of the Poynting vectors S1 and S2 must

remain in the positive x-direction through both media. Thus, Case 1 depicts the usual situation in

which M2 is a conventional positive index medium; however, if M2 is a medium supporting

propagating backward waves (LHM), then the wavevector k2 must be directed oppositely to the

Poynting vector S2. Therefore, refraction in media that support backward waves must be

described by the second case, in which power is propagated along the direction of phase advance,

and so is directed through a negative angle of refraction. Thus, M2 can be seen to possess an

effectively negative refractive index [3].

Fig. 1. 2: Refraction at a two-medium interface as determined by phase matching. The support of

backward waves by an LHM insists on negative refraction (Case 2).

3

1.1.3 Negative permittivity and permeability

While one often describes a material by some constant (frequency-independent) value of the

permittivity and permeability, in reality all material properties are frequency dependent. There

are several material models that have been constructed to describe the frequency response of

material. Because the magnetic field of an electromagnetic wave is smaller than its electric field

by the wave impedance of the medium in which it is propagating, one generally focuses attention

on how the electron motion in the presence of the nucleus and, hence, the baseic dipole moment

of this system are changed by the electric field. Understanding this behavior leads to a model of

the electric susceptibility of the medium and, hence, its permittivity. On the other hand, there are

many media for magnetic response of a material in a fashion completely dual to that of the

electric field using the magnetic susceptibility and, hence, its permeability. While the magnetic

dipoles physically arise from moments associated with current loops, they can be described

mathematically by magnetic charge and current analogs of the electric cases.

One of the most well-known material models is the Lorentz model. It is derived by a

description of the electron motion in terms of a driven, damped harmonic oscillator. To simplify

the description, it is assumed that the charges are allowed to move in the same direction as the

electric field. The Lorentz model then describes the temporal response of a component of the

polarization field of the medium to the same component of the electric field as (1.1).

iLiiLi EPPdt

dP

dt

d 0

2

02

2

(1.1)

The first term on the left accounts for the acceleration of the charges, the second accounts for the

damping mechanisms of the system with damping coefficient , and the third accounts for the restoring forces with the characteristic frequency . The driving term exhibits a coupling coefficient . The response in the frequency domain, assuming the engineering time dependence, is given by the expression as (1.2).

)()( 020

2

i

L

Li E

jP

(1.2)

With small loss

, the response is clearly resonant at the natural frequency . The

polarization and electric fields are related to the electric susceptibility as (1.3).

2

0

2

0

,)(

)()(

L

L

i

iLorentze

jE

P (1.3)

The permittivity is then obtained immediately as .

There are several well-know special cases of the Lorentz model. When the acceleration term

is small in comparison to the others, one obtains the Debye model as (1.4).

4

idiid EPPdt

d 0

2

02

2

2

0

, )(

d

dDebyee

j (1.4)

When the restoring force is negligible, one obtains the Drude model as (1.5).

iDiDi EPdt

dP

dt

d02

2

D

DDrudee

j

2,)( (1.5)

where the coupling coefficient is generally represented by the plasma frequency . In all

of these models, the high-frequency limit reduces the permittivity to that of free space.

Assuming that the coupling coefficient is positive, then only the Lorentz and the Drude

models can produce negative permittivity. Because the Lorentz model is resonant, the real part of

the susceptibility and, hence, that of the permittivity become negative in a narrow frequency

region immediately above the resonance. On the other hand, the Drude model can yield a

negative real part of the permittivity over a wide spectral range, that is, for .

Similar magnetic response models follow immediately. The corresponding magnetization

field components and the magnetic susceptibility equations are obtained from the polarization and electric susceptibility expressions with the replacement , . The permeability is given as [4].

To have negative refraction, the values of permittivity and permeability should be both

negative so that the metamaterial has to be designed carefully to share the frequency. These

metamaterial were demonstrated by Smith and Schultz[5] at the first time in microwave

frequency, and Valentine et al. demonstrated the structure in optical frequency[6].

5

1.1.4 Chiral metamaterial

A chiral medium is composed of particles that cannot be superimposed on their mirror

images [7]. A chiral medium has different responses for a left circularly polarized (LCP) wave

and a right circularly polarized (RCP) wave due to the intrinsic chiral asymmetry of the medium.

Also, there is cross-coupling between the electric field and magnetic field going through a chiral

medium. A dimensionless chirality parameter is used to describe this cross-coupling effect. The refractive indices of RCP and LCP waves become different due to the existence of .

In 2003, Tretyakov et al [8] discussed the possibility of realizing negative refraction by

chiral nihility. The authors first proposed the idea to fabricate a metamaterial composed of chiral

particles, such as helical wires. To get negative refraction for one of the circular polarizations, needs to be larger than . In natural materials such as quartz and sugar solutions, is generally much smaller than 1, while is generally larger than 1, so negative refraction is not possible in natural chiral materials. However, with chiral metamaterials, the macroscopic

parameters can be designed. The idea of chiral nihility is that, when and of a chiral medium are small and very close to zero, the chirality can make the refractive index for one circular

polarization to become negative, even when is small.

The metamaterial based on chiral nihility is a special case of chiral metamaterials. In 2004,

Pendry [9] discussed in general the possibility to achieve negative refraction in chiral

metamaterials. He analyzed the conditions to realize negative refraction in chiral metamaterials

and showed that they are simpler than for regular metamaterials, which require both electric and

magnetic resonances to have negative and negative . In chiral metamaterials, neither nor needs to be negative. As long as the chiral parameter is large enough, negative refraction can be obtained in chiral metamaterials. Pendry then proposed a practical model of a chiral

metamaterial working in the microwave regime with twisted Swiss rolls as elemental structures.

Chiral media belong to a wider range of bi-isotropic (BI) media. BI media [7] are

characterized by the following constitutive relations as (1.6) and (1.7).

HiED

000 )( (1.6)

EiHB

000 )( (1.7)

Depending on the values of and , the BI media are classified various, but the reciprocal chiral medium is defined by reciprocal , and chiral .

Consider the plane wave propagation in an isotropic chiral medium. Combining the above

constitutive relations with and the frequency-domain source-free Maxwell‟s equations, the following wave equation can be obtained for the electric field E:

)(2)()( 022

0 EkEkE (1.8)

)(2)()( 022

0 EkkiEkEkk (1.9)

6

where k is the wavevector in the chiral medium, is the free-space wavevector and

is the speed of light in vacuum.

For simplicity, and without loss of generality, we assume . Then the wave equation is simplified and k is solved:

)(0 kk (1.10)

where is the index of refraction of the medium without chirality. From this equation we

can write the chiral parameter as as

.

The eigenvectors, or the allowed solutions of plane waves in the chiral medium, can then be

obtained from . Then we get the following relations:

ikk

kki

E

E

x

y

)(

222

0

2

0

0

0

(1.11)

Therefore, the wavevector solution k+ corresponds to the eigenvector of the RCP wave and

k− corresponds to the eigenvector of the LCP wave. Here the handedness is defined as seen from

the source, or as looking in the direction of propagation. Define the index of refraction of

RCP/LCP waves as n±: then from the relation, the index can be obtained as (1.12).

nk

kn

0

(1.12)

The polarization plane of a linearly polarized light will rotate when it passes through a chiral

medium. Due to the chiral nature of the medium and the circularly polarized waves, the LCP and

RCP light interacts with the particles of the chiral medium differently. This causes the difference

in absorption and distortion of the two polarizations going through the medium, which is called

circular dichroism. Since the impedance of the chiral medium is the same for the two different

polarizations, the reflections of the two polarizations are the same [10].

1.1.5 Parameter retrieval of chiral media

Parameter retrieval is the procedure of obtaining the macroscopic parameters of a medium

based on the transmission and reflection coefficients (S parameters) from a planar slab of this

medium. For a homogeneous material, , and the refractive index n are intrinsic properties of the material and are irrelevant to the thickness of the slab. Thus one can choose a slab as thin as

possible to do the measurements, and thus obtain the parameters without ambiguity. However,

for a metamaterial slab, the structures are inhomogeneous and the smallest slab thickness is

limited by the unit cell size of the metamaterial [11,12,13].

For a chiral slab, the retrieval process is generally the same as regular metamaterials, but a

little more complicated. The refractive indices for the two eigensolutions (RCP and LCP waves)

need to be calculated [14]. Consider a circularly polarized plane wave normally incident upon a

7

homogeneous one-dimensional (1D) chiral slab of thickness d in vacuum, with refractive index

n± and impedance Z for RCP/LCP waves as Fig. 1. 3. The incident wavevector k0 is in the z

direction and the wavevector in the chiral slab for the RCP/LCP wave is k±. The electric and

magnetic field vectors are all tangential to the interfaces in this scenario.

Fig. 1. 3: The transmission and reflection coefficients of a plane wave incident upon a chiral slab

from the left

Suppose the incident RCP/LCP electric field has unit amplitude, the reflection coefficient is

given by R± and the transmission coefficient after the second interface is given by T±.

Multireflection happens inside the chiral slab and the transmitting and reflecting waves inside the

chiral slab are represented by coefficients T± and R± for RCP and LCP waves as Fig. 1. 3. Note

that the polarization state is reversed after reflection. Define normalized impedance of the chiral

slab as , where is the free-space impedance.

The tangential electric field and magnetic field are continuous at the first interface (x = 0):

RTR1 (1.13)

z

RTR

1 (1.14)

Similarly at the second interface (x = d):

TeReT

dikdik (1.15)

Tz

eReTdikdik

(1.16)

8

Note that , we can get the transmission and reflection coefficients from the above equations:

dink

dik

ezz

zeT

0222 )1()1(

4

(1.17)

dink

dink

ezz

ezR

0

0

222

22

)1()1(

)1()1(

(1.18)

R+ and R− are equal since the impedance for RCP and LCP waves is the same. If we denote

T and R as the transmission and reflection coefficients for the = 0 medium, we have

RR (1.19)

dkiTeT 0

(1.20)

The impedance and refractive index can be calculated from the above coefficients:

TTR

TTRz

2

2

)1(

)1( (1.21)

mRz

z

Tdk

in 2)

1

11(

1log

0

(1.22)

where m can be any integer.

The sign of the square root in equation (1.21) and the multi-branches in equation (1.22) need

to be chosen carefully according to the energy conservation principle, i.e. the real part of

impedance z must be positive, as well as the imaginary part of n.

Once z and n± are fixed, the other parameters can be identified subsequently as (1.23) and

(1.24).

2

nn

n (1.23)

2

nn

(1.24)

Therefore, negative refraction in chiral metamaterial can be achieved by the parameter though the permittivity and permeability are not both zero without frequency matching unlike the

conventional LHM [10].

9

1.2 Introduction to Surface Plasmon Lithography

1.2.1 Fundamental Physics of Surface Plasmon

Surface plasmons (SPs) are essentially electromagnetic waves trapped at a metallic surface

through their interaction with the free electrons of the metal as schematically illustrated in Fig. 1.

4. The recent discovery of extraordinary transmission through sub wavelength hole arrays on an

opaque metal film has sparked extensive interest in SPs among the scientific community

[15,16,17,18,19]. The observed far-field transmission through a silver hole array in the infrared

and visible region can be enhanced by orders of magnitude compared to that of a single hole.

This unusual enhancement is attributed to the excitation of SPs on the metal surface which

dramatically enhances the optical throughput via the sub-wavelength aperture. Since then,

numerous researches were focusing on the underlying physics and applications of SPs. The use

of SPs opens up the potential in nanoscale optical spectroscopy [20], surface-enhanced

spectroscopy, surface plasmon resonance sensing [21,22], and nanolithography [23,24].

The SP dispersion relation can be derived as below. Consider a plane surface of a semi-

infinite metal with the dielectric constant

, adjacent to a dielectric medium with the

dielectric constant :

02

2

1

10

zz kkD (1.25)

222)( zixi kkc

, 2,1i (1.26)

Therefore, the dispersion relation can be written as: [24]

2/1

21

21

ckx (1.27)

If

is assumed, a complex

can be obtained with (1.28).

2/1

2

'

1

2

'

1'

ckx , 2'

1

"

1

2/3

2

'

1

2

'

1"

)(2

ckx (1.28)

For real , one needs

and which can be fulfilled in a metal and also in a

doped semiconductor near the eigen-frequency; determines the internal absorption.

10

Fig. 1. 4: The charges and the electromagnetic field of SPs propagating on a surface in the x

direction are shown schematically.

From equation (1.27), the dispersion relation of SPs lies right of the light line indicating the

wave vector of SPs is always greater than that of light waves at a given frequency, as shown in

Fig. 1. 5. Due to the wave vector in z direction is imaginary; SPs are nonradiative

electromagnetic waves, which describe fluctuations of the surface electron density bounded to

the metal surface. Therefore, their electromagnetic fields have maximum strength at the surface

and decay exponentially into the space perpendicular to the surface. The most interesting

property of SPs is their dispersive behaviors which offer an access to much shorter wavelength

of surface waves than the excitation light wavelength. This extraordinary “optical frequency but

X-ray wavelength” property of SPs opens up potential of high-resolution imaging / lithography

at the length scale beyond the diffraction limit.

Fig. 1. 5: The SP dispersion relation. The dotted line represents the light line where the SP

dispersion curve always lies right of the light line.

11

To excite SPs using light, the momentum mismatch between the excitation light and the SPs

has to be compensated. As seen in the SP dispersion relation, the momentum of SPs is always

larger than that of a free space light. Therefore, a given additional momentum is needed to

couple light to SPs. Typically, a grating coupler and an attenuated total reflection (ATR) coupler,

are used to match the momentum mismatch, therefore, light can be converted into SPs and vice

versa. A grating can supply additional wave-vectors (G) which can compensate the momentum

mismatch of light and SPs. Fig. 1. 6 shows the coupling mechanism of a grating coupler. When

the wave-vector is matched as shown in the equation (1.29), the surface plasmons waves can be

resonantly excited using light excitation. kin is the incident wave-vector of the light and G is the

multiples of 2π/a where a is the period of the structure.

ankGkk ininsp

2 (n: integer) (1.29)

Fig. 1. 6: Momentum matching through the use of grating coupler

12

1.2.2 Plasmonic Nearfield Lithography

Nearfield scanning optical microscopy (NSOM) is based on AFM(Atomic Force

Microscopy) technology and uses a scanning probe to scan in the nearfield of the sample and this

has been applied extensively in the studies of biology, material science, and data storage. In spite

of NSOM‟s capability of a resolution beyond the diffraction limit of the light, its applications are

somewhat limited by the strong attenuation of the light transmitted through the sub wavelength

aperture because of the characteristics of an evanescent wave, making it inappropriate for the

applications such as high-speed probing and high-throughput nanolithography. Obviously, a sub

wavelength optical spot size with high light intensity is very helpful for the aforementioned

applications. The application of surface plasmon coupling with focusing structures can enhance

the intensity of transmitted light through a sub-diffraction-limited spot which enables probing the

sample in the nearfield with high resolution and high efficiency [25].

To excite surface plasmon from the incident light, a set of grating as Fig. 1. 7 are fabricated

on a tapered fiber tip over aluminum thin film. The grating‟s dimension and period are calculated

based on additional wavevector to match surface plasmon wavevector with the incident light

wavevector and the constructive interference at the center aperture.

Fig. 1. 7: Tip side view and cross-sectional view of geometry ((a) and (b)) and simulated

intensity of the nearfield ((c) and (d)) of the plasmonic structures. The scale bars is 500nm.

13

The following 3-D simulation result which is calculated by CST Microwave Studio shows

the optical field intensity of the plasmonic structures normalized to the intensity without rings as

Fig. 1. 7. The peak enhancement factor of about 36 is observed at 365nm, which is exactly our

designed UV wavelength. The cross-sectional view of the intensity profile at the plane near the

aperture shows a very good field confinement with a spot size of ~100nm along the x direction

and ~85nm in the y direction. This asymmetry produced by the symmetric structure is due to the

horizontal polarization of the incident light.

As shown in Fig. 1. 8, this plasmonic NSOM can also be used in nanolithography

applications. During the lithography process, a laser beam at the wavelength of 365nm was

coupled into the NSOM plasmonic tip to expose a positive photoresist. A best pattern linewidth

of 80 nm was demonstrated. Similar exposure results through the tip without the plasmonic

structures were only obtained by using 10 times higher input power.

This NSOM based approach allows us to utilize a light source with longer wavelength to do

nanolithography. However, it still cannot overcome the limitation of scanning probe based

lithography approaches due to its slow scanning speed. In order to meet the requirement of real

world applications, the scanning speed needs to be at least one million times faster.

Fig. 1. 8: The configuration of nearfield lithography and AFM image of the photoresist after

near-field scanning exposure using the plasmonic structures

14

1.3 Dissertation Organization

This dissertation is focusing on designs and fabrications of metamaterial and also nanoscale

manufacturing method. Metamaterial parts contain the applications from microscale for THz to

nanoscale for infrared, especially the fabrications are emphasized to realize them. Surface

plasmon lithography part contains the fundamental techniques to achieve next generation high

resolution lithography method.

This dissertation is composed of 7 chapters. The first chapter provides the background of

this dissertation which contains the fundamental of metamaterial like definition of negative

permittivity, permeability, and refraction. Also, the basic information of surface plasmon

lithography is followed.

Chapter 2 describes THz chiral metamaterial which can achieve negative refraction without

matching negative values of both permittivity and permeability. This chapter provides the design,

fabrication, and experimental results of the world‟s first demonstration of chiral metamaterial

and, switchable chiral metamaterial which can change chirality dynamically based on the first

design.

Chapter 3 is dedicated to the design, fabrication, and experimental results of asymmetric

isotropic metamaterial which accomplished to have negative refraction independent to the

direction of polarization by controlling symmetry of the structures. In addition, it is shown that

random distributed structure can represent negative refraction beyond any research in

metamaterial.

Chapter 4 describes three different applications of metamaterial. The first topic is the

method to measure a mode volume by a far field method and the second topic is detection of a

single molecule by far field measurement using split ring resonator. This chapter informs that

metamaterial has variety of applications as well as negative refraction.

Chapter 5 focuses on surface plasmon lithography which can overcome beyond diffraction

limit for high resolution maskless lithography method. This chapter covers the designs and

fabrications of each key component i.e. air bearing slider, surface plasmonic lens, and disk, and

experimental results.

Chapter 6 is dedicated to show the details of the fabrication processes which are used in the

topics of this dissertation. The purpose of each process and the recipes are provided. This chapter

provides from microscale fabrication method for THz chiral metamaterial to nanoscale

fabrication methods for asymmetric isotropic metamaterial and other applications.

Finally, Chapter 7 summarizes the work in this dissertation and discusses the potential of

metamaterial and surface plasmon lithography from a view of nano-manufacturing in future

applications.

15

Chapter 2 Chiral Metamaterial

2.1 Terahertz Chiral Negative-Index-Metamaterials

2.1.1 Background

Terahertz is a unique frequency range with many important applications such as security

detection and gas phase molecule sensing [26]. However, the devices for manipulating the

terahertz wave are considerably limited. Consequently, the development of artificial materials

with unusual optical properties at this frequency region is especially important. The recent

development in metamaterial research has led to the achievement of devices with unusual optical

functionalities at terahertz frequencies. [27, 28, 29, 30, 31] The realization of NIM(Negative

Index Metamaterial)s in the terahertz regime may lead to discoveries of new physics and novel

applications. Recently, a chiral route to achieve negative refraction has been proposed. [32] This

approach brings great robustness to the design of NIMs since the same structure offers both

electric and magnetic resonances at the same frequency; in addition, negative refractive index

can be realized without the requirement of simultaneous negative permittivity and permeability

due to the presence of strong chirality. Interestingly, chiral metamaterial can be designed such

that the negative refraction works only for one circularly polarized wave, while for the other

circular polarization the index is positive. This gives rise to some interesting phenomena that

normal NIMs do not exhibit, such as negative reflection for circularly polarized wave incident on

a mirror embedded in such a medium [33]. Especially, in the special case where two circularly

polarized waves having refractive indices of the same amplitude and opposite signs, the light

incident on the mirror would be reflected back at exactly the same direction. This resembles the

phenomenon of time reversal similar to the case of light reflected from a phase conjugate mirror,

but without involving nonlinearity. [34]

2.1.2 Design of THz Chiral Metamaterial

Various designs for chiral negative index metamaterial have been proposed recently.[31,35,

36] However, due to the complexity of the 3D metamaterial geometry, experimental realizations

of these chiral NIMs at the terahertz and optical frequencies have not been available yet.

Although metamaterials with considerable optical activity were recently demonstrated at the

microwave and optical frequencies using a simplified multilayer planar design, but no evidence

of negative refractive index has been shown in these works.[37,38] Here, we design and

experimentally demonstrate a chiral metamaterial operating in the terahertz region which

exhibits a huge chirality and negative refractive index for circularly polarized waves. The

schematic of a single chiral element is shown in Fig. 2. 1 (a). The metallic chiral structure is

equivalent to a micro sized inductor-capacitor (LC) resonant circuit, with the inductor formed by

the loop and the capacitor formed between the two bottom metal strips. Oscillating current

flowing through the metal loop can be excited by either an electric field across the gap or a

magnetic field perpendicular to the loop, which in turn generate strong electric and magnetic

responses. [39] Therefore, this structure can be considered as the combination of an electric

dipole and a magnetic dipole, as indicated in Fig. 2. 1(b). Since the electric and magnetic dipoles

16

share the same structural resonance, the excitation of one dipole would inevitably lead to the

excitation of the other. Because the angle between directions of the two dipoles is small (~30o), a

strong chiral behavior is expected.

Fig. 2. 1: (a) The schematic of the chiral structure, with some of the dimensions indicated in the

figure: L =18m, h=5m, r=1.6m, w=4.4m, g=2.3m. The thicknesses of the bottom gold

strips and the top gold bridge are 0.6 and 0.3 m, respectively. (b) The top view of the chiral

structure, which functions effectively as an electric dipole and a magnetic dipole forming an

angle =29.2o.

2.1.3 Fabrication

A number of fabrication methods have been employed to realize complicated three

dimensional structures in the micrometer and even nanometer scales, such as micro-stereo

lithography and two photon lithography.[36, 40, 41] However, from the manufacture point of

view, planar micro-fabrication processes would be highly desired. In this work, we have

fabricated large scale (2cm by 2cm) chiral metamaterials using standard semiconductor

processing technologies. The fabrication procedure is as follows. First, a thin layer of thermal

silicon dioxide is deposited on a silicon substrate with very low doping level, followed by metal

sputtering of aluminum with 600nm, which will later serve as the electrode for electroplating

process. [Fig. 2. 2(a)] Optical lithography and liftoff were carried out to deposit parallel metal of

gold strip pairs on top of the aluminum layer.[Fig. 2. 2 (b)] To ensure that the chiral medium has

an eigen-mode of circularly polarization for normal incident waves, the strip pairs are arranged

with a fourfold rotational symmetry within the unit cell. For the lift off process, Microchem

LOR10A and Shipley S1818 were used to have vertical negative angle of resists. This prevents

side wall deposition and results in accurate dimensions of strips. SOG(Spin-on-glass) was then

spun and thermally cured onto the sample to protect aluminum surface because aluminum is easy

to be damaged by following chemicals. Optical lithography, oxygen RIE etch are following to

open the area of pillars. A sacrificial SU8 layer was then spun onto the sample, followed by

lithography and plasma RIE etching to define holes and match its thickness in the sacrificial

17

layer on top of the metal strips. [Fig. 2. 2 (c)] Next, gold electroplating process was performed to

deposit metal inside the holes, resulting in gold pillars with a height of approximately 5 m. [Fig.

2. 2 (d)] Optical lithography and liftoff step were carried out again to form the gold bridge

connecting the gold pillars. [Fig. 2. 2 (e)] The sacrificial SU8 layer was then removed using

oxygen plasma reactive ion etching, and the SOG and exposed aluminum were selectively etched

away using a wet etching process, resulting in the final chiral metamaterial [Fig. 2. 2 (f)], with a

total patterned area about 2cm by 2cm. The SEM images of the structure are shown in Fig. 2. 2

(g) and (h). The details of each process including recipe is introduced in chapter 6.1.1. We note

that the metamaterial design and fabrication process can be easily transferred to sub-micron scale

by replacing optical lithography with electron beam lithography. Indeed, by using Ebeam

lithography and layer-by-layer fabrication, a three-dimensional magnetic metamaterial has been

recently realized which operates at the infrared frequencies. [42]

Fig. 2. 2 (a)-(f): the schematics of the fabricaton procedures for the chiral metamaterials. (g), (h):

the SEM images of the chiral metamaterials at tilted angle. The size of the unit cell is 40 m by

40 m. The scale bars in (g) and (h) represent 100 m and 20 m respectively.

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2.1.4 Experiment

In order to obtain the transmission amplitude and phase, terahertz-time domain spectroscopy

(THz-TDS) was carried out, as shown in Fig. 2. 3 (a) [43,44].The sample is placed midway

between the transmitter and receiver modules at the waist of terahertz beam. Two free standing

metal wire polarizers were employed, one in front of and one after the sample to characterize the

transmission of the same polarization state as that of the incident wave t1 (P1//P2) and that of the

perpendicular polarization state t2 (P1P2). The complex coefficients for the transmissions can

be obtained by taking the Fourier transform of the time signal, which is calibrated over a bare

silicon wafer. Care was taken for the extraction of the phase information since the reference

silicon wafer has slightly different thickness from the substrate on which the chiral metamaterials

were fabricated. Consequently, instead of the absolute transmission phase, relative phases which

solely represent the resonance feature were derived from the measurements. Due to the four fold

rotational symmetry of the sample, the transmission properties of the sample do not rely on the

orientation of the sample, which was confirmed by the measurements. For left and right

circularly polarized beams tL and tR can be inferred by,

21

21

ittt

ittt

R

L

(2.1)

Fig. 2. 3: The time domain terahertz spectroscopy setup to characterize the transmissions of the

chiral metamaterials. The photoconductive switch-based THz-TDS system consists of four

paraboloidal mirrors arranged in an 8-F confocal geometry, enabling excellent terahertz beam

coupling between the transmitter and receiver and compressing the beam to a frequency

independent beam waist with a diameter of 3.5 mm as well. The THz-TDS system has a usable

bandwidth of 0.1-4.5 THz (3 mm-67m) and a signal to noise ratio over 4 orders of magnitude.

M2

M1

M3

M4

Fs

L

aser

Receiver

Si

Lens

Fs

Laser

Transmitter

Si Lens

Sample

Polarizer

2 Polarizer

1

45º 45º x

y

19

2.1.5 Results and discussion

The measured transmission amplitudes and phases for t1 and t2, which are calculated by

taking a Fourier transform of the signal in the time domain, are shown in Fig. 2. 4. A resonance

occurs around 1 THz, exhibiting a dip in t1 and a peak in t2, indicating a strong chiral behavior

that leads to conversion of a large portion of the energy to polarization perpendicular to that of

the incident wave. The resonance is accompanied by a steep slope in the relative phases of the

transmissions. Using equation (2.1), the transmission for left circular polarized beam (LCP) and

right circularly polarized beam (RCP) are derived and shown in Fig. 2. 4 (c) and (d). The

resonance for LCP shows a much more pronounced feature than that of RCP. At the resonance,

the dip of LCP transmission almost approaches zero (

20

equation and taking into account the substrate. The effective indices for both LCP and RCP are

shown in Fig. 2. 5(e). The refractive index for the left handed wave shows a strong modulation at

the resonance frequency, and reaches negative values over the frequency range between 1 to 1.3

THz, with a minimum index below -5. This, to the best of our knowledge, is the first

demonstration of negative index chiral material in the terahertz and above. On the other hand, the

refractive index for the right handed wave shows very small modulation at the resonance

frequency and remains around 4 over the whole frequency range. This is due to the cancellation

of the resonant feature of refractive index between the contribution from permittivity

/permeability and that from the chirality. Far from the resonance, the difference between the

refractive indices of RCP and LCP tends to diminish, verifying the significant role that the

structural LC resonance plays in achieving the strong chiral behavior. Finally, the effective

permittivity and permeability are calculated and shown in Fig. 2. 5 (f). Both of them exhibit a

Lorentzian-like lineshape. While the permittivity reaches negative values at resonance, the

magnetic resonance is not strong enough to achieve negative permeability. Nevertheless, the

existence of strong chirality in the metamaterial lifts the requirement for simultaneous negative

permittivity and permeability.

21

Fig. 2. 5: (a)-(d): the FDTD simulation of the transmission amplitudes and relative phases for the

chiral metamaterials, with the same color conventions as in Fig. 2. 4. (e) The real part of

refractive indices for right handed (red) and left handed (blue) circularly polarized waves. (f) The

real part of effective permittivity (blue) and permeability (red).

In summary, the first terahertz chiral metamaterials exhibiting very strong circular

dichroism and opposite signs for the refractive index of the circularly polarized waves around the

resonance frequency of 1 terahertz has been demonstrated. The fabrication of the sample relies

on a planar process which combines optical lithography, lift-off and electroplating, and is

compatible with standard semiconductor process. The layer-by-layer nature of fabrication

techniques make it possible to manufacture a three dimensional array of the chiral elements, thus

enabling further investigations of the interesting bulk properties of the terahertz chiral

metamaterials. The terahertz metamaterials, when combining with the optical or electrical tuning

techniques, would lead to a terahertz device which can control the polarization of terahertz wave

dynamically.

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2.2 Photo-Induced Chirality Switching in Metamolecules

2.2.1 Background

Chirality, a terminology defining structures that lack any mirror symmetry planes, is

ubiquitous in nature, ranging from organic molecules, polymers to crystals such as quartz.

Chirality is very important in biology and medical sciences since most biomolecules – the

building blocks of life, are chiral and enantiomerically pure, such as D-saccharides, D-

nucleotides and L-amino acids. Molecules of opposite handednesses, though having the same

physical and chemical properties, may exhibit dramatically different physiological properties,

such as odour, receptor binding, and pharmacological effect. In terms of optical properties, it is

well known that chiral molecules interact differently with light of left h